MKIIIB-CTD - IMPROVING ITS SYSTEM OUTPUT

In the Neil Brown Instruments' MKIIIB-CTD (conductivity-temperature-de pth profiler), the system's digital outputs for the three basic measur ements of temperature, conductivity and pressure typically show some s mall amplitude deviations from smooth calibrations which should be cor rected for to achieve high accuracies, as required, e.g. within the Hy drographic Program (WHP) of the current World Ocean Circulation Experi ment (WOCE). These deviations show up as (i) a strong nonlinearity or even discontinuity of several mK close to 0 degrees C in temperature o utput leading to too high subzero temperatures; (ii) a jump of order 0 .002 mS cm(-1) in conductivity output when passing the half-range valu e 32.768 mS cm(-1), which causes jumps in the relation of potential te mperature and salinity; and (iii) errors in pressure measurements of u p to 4 dbar due to mechanical hysteresis and both static and dynamic r esponses to temperature changes. The existence of these effects is dem onstrated, and methods to reduce the associated errors are suggested.